This invention relates generally to power on reset circuitry, and more particularly the invention relates to a power on reset circuit which has a trip point or deassertion point after power reaches an operational voltage level.
Electronics systems typically employ a number of integrated circuit components all operating from a common voltage power supply. When a system is initially turned on, a supply voltage can ramp up to full voltage over a period of time from less than a microsecond to over a hundred milliseconds. While the supply voltage is ramping up, reset circuits within the integrated circuits initialize the integrated circuits by initializing flip flops, registers and latches for example, so that the circuit has correct start up configuration when the power supply voltage reaches an operational level for all parts of the circuit required to work on start up.
By its nature, a power on reset circuit is usually process sensitive with the deassertion or trip point being a function of the threshold voltage, semiconductor body effect, and to a lesser extent the other characteristics of the transistors in the integrated circuits. Thus, there will always be some spread, typically more than a volt, for the deassertion level of the reset circuit. Too high an average level is undesirable because for some process parameter corners and temperatures the trip point may drift above the supply voltage, V.sub.cc, and thus the reset circuit will never go low. Further, if the trip point is too high, the reset circuit can become sensitive to supply noise while the device is in operation and needlessly reset. Delay circuits have sometimes been used to ensure that the trip point is above the operational voltage level of the power supply. However, the supply ramp can vary greatly from application to application as noted above. If the deassertion level is too low then the reset circuit may turn off before the rest of the circuitry is operational whereupon the device could start up incorrectly in the wrong state.
The present invention ensures that the power on reset circuit always turns off or fires after the integrated circuit becomes operational.